Chromium coating is widely used as a surface coating for metal articles because of its high hardness value, attractive appearance and superior wear and corrosion resistance. Traditionally, chromium deposition is accomplished by electrodeposition from a chromium plating bath containing hexavalent chromium ions as a source of chromium. The process is highly toxic in nature. Lots of efforts have been made to develop alternative coatings and coating processes to replace the use of hexavalent chromium in electroplating. Among those alternative processes, trivalent chromium electroplating seems to be attractive due to convenience of fabrication through the use of environmental friendly and non-toxic chemicals and ability to produce a bright chromium deposit. However, an industrial scale process giving a hard and corrosion resistant chromium deposit through an aqueous trivalent chromium solution is still missing. Among the industry, there is a hectic need for a well manageable and easy to use chromium based coating process to replace the current use of hexavalent chromium in coating.
Decorative chrome is designed to be aesthetically pleasing and durable. The thickness of decorative chromium coating is generally between 0.05 and 0.5 μm. There has been a strong movement away from hexavalent decorative chromium baths to new trivalent chromium baths. The trivalent form of chromium is considered to be less toxic.
Hard chrome is used to reduce friction, improve durability through abrasion tolerance and wear resistance, minimize galling or seizing of parts, expand chemical inertness to include a broader set of conditions, and as bulking material for worn parts to restore their original dimensions. Hard chromium coatings tend to be thicker than decorative chromium coatings. The thickness of hard chrome can be as high as 200-600 μm. Due to its thickness, the hardness of hard chrome is usually over 700 HV. Today, hard chrome is almost exclusively electroplated from hexavalent chromium baths because of difficulties in reaching desired wear resistance and hardness by using trivalent chromium baths.
Many chromium plating processes of prior art are not capable of producing coatings with a Vickers microhardness value of 2000 HV or more. Further defects of the known chromium-based coatings are their inadequate wear and corrosion resistances. Chromium coating as such is very brittle in character. The number of cracks and microcracks in a chromium coating increases together with the thickness of the coating, thus impairing the corrosion resistance of the coating.
Deposition of nickel, either by electroless plating or electroplating, has also been proposed as an alternative to hard chrome. Drawbacks of nickel plating include deficiencies in hardness, friction coefficient, wear resistance, corrosion resistance and adhesion. Nickel plating and hard chrome are not interchangeable coatings. The two have unique deposit properties and, therefore, each has its distinct applications.
It is well known in the art that the hardness of a chromium coating can be improved, to some extent, by thermal treatment. According to P. Benaben, An Overview of Hard Cromium Plating Using Trivalent Chromium Solutions, http://www.pfonline.com/articles/an-overview-of-hard-chromium-plating-using-trivalentchromium-solutions, the microhardness of a chromium deposit as-plated is about 700-1000 HV100. By a heat treatment at 300-350° C. the microhardness of trivalent Cr can be increased up to about 1700-1800 HV100. At higher temperatures the hardness of the Cr deposit tends to decrease. Adhesion of a trivalent Cr layer is known to cause problems. The process chemistry of known trivalent Cr baths is often very complicated and hard to manage.
U.S. Pat. No. 5,271,823 A discloses a method for providing a wear resistant Cr coating on a metal object, including the steps of electrodepositing a coating made solely from trivalent Cr ions and devoid of hexavalent Cr ions on the object and heating the coating to a temperature of at least 66° C. for at least 30 minutes.
U.S. Pat. No. 5,413,646 A discloses a method for electroplating a workpiece, comprising the steps of providing a plating bath comprising trivalent Cr produced by reducing a Cr(VI) compound to Cr(III) compound with methanol or formic acid, providing an anode in the plating bath, placing the workpiece in the bath to act as a cathode, electroplating a chromium and iron metal layer onto the workpiece, and heating the workpiece from about 316° C. to about 913° C. for a sufficient period of time to harden the workpiece while retaining or increasing the hardness of the chromium alloy plated on the workpiece.
U.S. Pat. No. 6,846,367 B2 discloses a heat-treating method for improving the wear and corrosion resistance of a chromium-plated steel substrate, comprising the steps of plating a chromium layer onto an steel substrate and heating the chromium-plated steel substrate in an oxidizing gas environment at above atmospheric pressure to form oxidized layers containing magnetite (Fe3O4) on the surface of the steel substrate, the surface of the steel substrate being partly exposed to the air through penetrating cracks formed in the chromium layer.
U.S. Pat. No. 7,910,231 B2 discloses a method for producing a coated article comprising a substrate and a coating on the substrate, the coating comprising chromium and phosphorus, Cr and P being present in at least one of the compounds CrP and Cr3P. Phosphorus is brought into the coating as a part of the chromium solution, and the maximum hardness that can be reached after a heat treatment is 1400-1500 HV. The coating lacks nickel, as do all the other chromium coatings referred to above.
The hardness, friction coefficient, wear and corrosion resistance of known trivalent Cr coatings are not sufficient to satisfy the demands of industry. The coating processes of prior art are not capable of producing coatings with a Vickers microhardness value of about 2000 HV or more.
Apparently, there is a need in the art to find a cost-effective trivalent chromium-based electroplating method, which is able to yield such utmost mechanical properties that enable replacement of hexavalent chromium baths in industrial use.